Flowmeters are employed to monitor the rate at which a gas travels through a conduit. High performance flowmeters are required for specialized applications such as the monitoring of the flow of anesthetic gas mixtures into and out of a surgical patient.
When a gas flows through a conduit it loses pressure head due to wall friction. This pressure loss, called the frictional pressure drop, is unrecoverable (permanent) and contributes to the flow resistance of the conduit. Additionally, if the gas encounters a mechanical moving obstruction in the conduit which is rotated or moved by the passage of the gas, then the gas loses an amount of energy equal to the work needed to move the mechanical component. This energy loss also manifests as an unrecoverable pressure drop.
Many volumetric flowmeters are equipped with transducers which are passively driven by the gas flow. Such transducers therefore suffer from a loss of energy as a result of a permanent pressure drop.
Active transducers are devices in which the transducer is driven by an external force such as a motor. When a change occurs in the flow rate, there occurs a mismatch between the speed of the transducer and the flow of gas therethrough. As a result, a differential pressure drop is generated between the inlet and outlet regions. Once the pressure drop is detected, the flowmeter acts to change the speed of the transducer and thereby eliminate the pressure drop.
These principles have been demonstrated previously and are described in a number of patents. For example, Huebotter, U.S. Pat. No. 1,797,233 discloses a fluid-metering gear pump employing two-lobed intermeshing displacement members which are rotatably mounted within a casing. The speed of the pump is controlled so as to maintain a near zero differential pressure between the inlet and outlet regions and thereby provide a flow rate measurement.
Holzem, U.S. Pat. No. 3,633,420 discloses a gear pump driven by a motor control system in response to a differential pressure across the pump. The gear pump operates with a pressure difference sensor system containing both a proportional and an integral pressure difference sensor.
Holzem et al., U.S. Pat. No. 4,193,299 discloses a positive displacement fluid flow metering device employing an additional bypass channel containing a pressure sensor and a servo motor which is responsive to the pressure sensed by the sensor to thereby control the operation of the device.
Holzem et al., U.S. Pat. No. 4,305,302 discloses a flow transducer element which incorporates rotors. The flow measuring device incorporates two differential pressure sensors of different sensitivity and means for automatically correcting for zero drift at periodic time intervals.
Groffo U.S. Pat. No. 3,550,426 discloses a cylindrical transducer having a series of vanes on its surface. A pair of differential pressure measurements are taken in each channel of a split flowmeter to compute a result indicative of the deviation of actual meter response from its factory calibration.
Such known flowmeters are disadvantageous because of extended response times in which a considerable time elapses between the generation of a pressure drop and a change in the speed of the transducer. Since the change in speed of the transducer is a function of the torque of the motor and the mechanical load of the transducer, a low inertia transducer would reduce the response time.